An anchor windlass typically has a wheel either constituted as a capstan, sprocket, drum, or barrel that can be rotated at low speed with high torque to haul in an anchor. Normally to drop anchor the sprocket is allowed to rotate freely, although it is also normally desired to drive the sprocket in both directions to pay out a sufficient anchor line or rode to obtain the desired scope.
A typical such windlass has a wheel which is rotatable and limitedly axially displaceable on the windlass shaft. An abutment is axially fixed on the shaft to one side of this wheel and an entrainment element on the other side of this wheel is axially shiftable but rotationally coupled to the wheel. Mechanism normally including a nut threaded to the shaft can axially press the entrainment element forwardly against the wheel to wedge this wheel between the entrainment element and the abutment, thereby rotationally coupling the wheel to the shaft. Opposite rotation of the nut releases the wheel to rotate freely on the shaft. In such an arrangement a pawl can be used to rotationally arrest the nut, so that when the shaft is rotated in one direction the nut will move forwardly to lock the wheel on the shaft, and when the shaft is rotated in the opposite direction the nut will be screwed backwardly.
It is also known to form the entrainment element and nut integrally, so that the entrainment element is in effect simply screwed into tight contact with the wheel. Such an arrangement has the advantage of considerable simplicity, and is capable of exerting enormous axial forces on the wheel to lock it rotationally tightly in place, as the wheel must be able to exert considerable force on the anchor line to haul anchor.
These systems have several disadvantages. In the first above-described system the mechanism is subject to normally very abrupt forces during the coupling-up and uncoupling of the wheel. Thus the mechanism must be made extremely robust, and even so can be counted on to need periodic expensive servicing. In the other arrangement considerable forces must be brought to bear not only to lock the wheel on the shaft, but also to uncouple the wheel from the shaft. The considerable frictional forces lead to rapid wear and, hence, once again make the system relatively service-prone.